Between the sputtering U.S. economy and the turbulent debt crisis in Europe, investors could be forgiven if China’s growth trajectory has not been top of mind lately. That can change in a hurry though, and with recent economic data signaling slower growth the bearish voices on China are getting louder.

To John Canally, investment strategist and economist at LPL Financial, the concerns about China could be “the worry du jour of early 2012.”

A real growth collapse in China – say a breakdown to a 5% annual GDP expansion – would still have the economy there growing at a faster clip than anyone expects the U.S. to, but it will have widespread consequences for a global economy that has leaned heavily on demand out of China since the financial crisis.

Whether it is demand for commodities – check out the slide in copper prices over recent months if you’re still in the camp that doesn’t believe China is slowing at all, says Cannally – or consumer goods, China has been a ready and willing buyer for Western sellers at a time when developed market consumers and businesses are in the midst of rather drastic belt-tightening.

Back in September, Morningstar analysts addressed what the firm calls “an unsustainable investment boom” and raised concerns about two trends that most China bulls cite as the backbone of continued blockbuster growth: unprecedented urbanization and the rise of the middle class.

China’s baseline definition of “urban,” Morningstar says, may overstate the capacity for further migration to major cities. “Four of the 10 largest U.S. cities would fail to meet the test,” of being declared “urban” by Chinese measures. Meanwhile, the track record of fast-growing economies transitioning from investment to consumption does not show many smooth handoffs. In the case of Japan and South Korea, “consumption growth rates actually declined in the post-boom decade, resulting in headline GDP growth roughly 40% what had been achieved in the boom decade.” With some of the hope for a soft landing in China pegged to more spending from consumers, a shortfall there could make for a bumpy ride.

It is not just China’s predecessors that illustrate the vast amount of capital-intensive, fixed-investment growth that has driven China’s rise over the past decade plus. Morningstar notes that even among its contemporaries, Brazil, Russia and India (together the four are known as the BRICs), the composition of China’s growth is a clear outlier.

Gross capital formation as a share of output in China clocks in at 42% over that span, compared to lower figures in Brazil (17%), Russia (21%) and India (31%), while the contribution of household consumption – just 34% of 2010 GDP in China – is dwarfed by that in Brazil (61%), Russia (51%) and India (57%).

With concern about the makeup of Chinese growth, and its sustainability, mounting, investors are right to think about how their portfolios are exposed to China. Some businesses or investments are indirectly exposed, and would only see the impact of slower Chinese growth only in so far as it hurts economic output elsewhere. Other companies have direct exposure to China, drawing a substantial amount of revenue from the country.

The latter group includes the likes of casino operators Wynn Resorts and Las Vegas Sands, darlings of the stock market in recent years thanks to their rapidly-growing gaming operations in Macau, but certainly vulnerable to any slowdown in China.